Antennas are often used for radiating a signal into free space for receipt by another device. Different antennas radiate signals differently. However, all antennas require that the radiated signal make a complete circuit with the signal source. For example, a portion of the signal radiating from an antenna formed by a single wire, i.e., a monopole antenna, will electrically couple with the electrical ground surrounding the antenna. The electrical ground is referred to as the “ground plane.” Certain antenna types, such as inverted “F” or bent monopoles are susceptible to interference by other concurrent signal currents such as digital circuits, clocks and other fast switching signals. Often, shared return paths for the antenna currents and high speed digital signals interfere with one another. The use of different antenna configurations as well as shielding techniques can minimize the effects of the ground plane interference.
One antenna configuration that may reduce ground plane interference is a dipole antenna. A dipole antenna includes a feeder conductor and a resonant conductor (also called an antenna conductor). Typically, the resonant conductor has an overall length that is approximately equal to a half wavelength. The feeder conductor provides signals to the resonant conductor. The resonant conductor may be envisioned as two conductors shorted together at the center. A parallel arm conductor may be connected to the resonant conductor to establish a desired resonance. The resonant conductor length is typically an odd number of half wavelengths long, which is shorter in length than the parallel arm conductor. The parallel arm conductor may couple parasitically to the feeder conductor, and re-radiate a signal if properly stagger-tuned to the resonant conductor. The resonant conductor is supplied by a signal source at a pair of feed points from the feeder conductor near its center, and the resonant conductor may connect to the feeder conductor near to one of feed points.
The signal source provides a signal having a current and a voltage. The length, diameter and volume of the antenna conductors affect the impedance and bandwidth of the antenna. At the center of the feeder conductor, the current value is at a maximum and the voltage value is at a minimum. This results in a low impedance at the center of the feeder conductor. By matching the impedance of the antenna, called “gamma matching,” to the input impedance of the signal source, the optimum power transfer between the feeder and the signal source and the maximum operational efficiency may be attained for signal transmission and reception. The dimensions (conductor length and diameter (i.e. volume)) of the resonant conductor and the resonant conductor's placement on the feeder conductor are selected by gamma matching. Typically, most systems use a balun to provide a balanced current that is out of phase.